(1) Field of the Invention
The present invention relates in general to automotive lamp units, and more particularly to automotive lamp units of a type which is designed to prevent the collection of moisture on the inner surface of the lens and to maintain the interior of the lamp unit moisture free.
(2) Description of the Prior Art
Hitherto, various kinds of automotive lamp units, such as head lamp, tail lamp, fog lamp, etc., have been proposed and put into practical use. In order to protect the interior of the lamp unit from raindrop, dust or the like, some of the conventional lamp units are of a so-called `sealed type` which is constructed to hermetically close the interior of the lamp unit from the atmosphere.
However, from the practical point of view, the sealed lamp unit is forced to breathe through any incompletely sealed portions because of the unavoidable expansion-contraction thereof originating from the temperature change of air therein due to ON-OFF operation of the lamp proper. Thus, in fact, the interior of the sealed lamp unit is contaminated but in small amounts, with the surrounding air. Furthermore, some of the sealed lamp units are equipped with an air breather means to prevent breakage thereof originating from the expansion-contraction phenomenon.
Thus, even in the sealed type, the interior of the lamp unit is contaminated with the surrounding air. Due to moisture in air, the inner surface of the lens of the lamp unit is collects moisture thereby deteriorating the light transmission ability thereof.
For solving the above-mentioned undesirable moisture collecting phenomenon, Japanese patent application First Publication No. Sho 49-48788 has proposed a measure of using a desiccant pack in the lamp unit. In this measure, the desiccant pack contains silicagel and is bonded to the inside surface of the lamp unit by means of a suitable adhesive. However, this measure fails to provide satisfaction because the desiccant pack tends to come off from the inside surface of the lamp unit when a considerable shock is applied thereto during moving of the vehicle. Furthermore, in this measure the heat from the electric bulb is directly applied to the desiccant pack thereby to deteriorate the moisture adsorption ability of the same.